This invention relates generally to two-way cable television (CATV) systems and is particularly directed to the transmission of data from the headend to a plurality of subscribers in a two-way CATV system.
CATV systems typically include a central master source of television programming information termed a headend which imparts programs, or groups of programs at different assigned frequencies, to a network of cable connections which, through various distribution and selection network components, ultimately terminate in a branched out plurality of subscriber terminals typically in residence or commercial establishments. CATV program signals are broadcast from the headend to individual subscribers in a "downstream" direction over different assigned carrier frequencies typically from about 50 to 450 MHz. The CATV headend not only transmits television programming information downstream, but may also transmit data in the form of, for example, subscriber address and authorization information, which allows the stored program decode authorization status of each subscriber to be individually controlled from the headend. Frequently, such data is encoded in the vertical blanking interval (VBI) of a transmitted CATV program signal. The program decode authorization status stored in each subscriber's terminal is compared with a program code also encoded in the VBI of the received CATV signal to establish whether or not the subscriber is authorized for decoding the received program signal.
In a two-way CATV system each subscriber is able to transmit signals back to the CATV headend. These subscriber-originated signals may include program purchasing requests, opinion poll responses, and CATV converter status information. These upstream signals typically make use of out of band frequencies in the 5-32.5 MHz band. The upstream signal distribution network is in the form of a "merging tree topology" in which the signals generated by many sources, or subscribers, converge and are transmitted on a single transmission line back to the CATV headend. Such techniques as signal multiplexing and frequency diversity have been proposed and are utilized to accomodate large numbers of subscribers in two-way CATV systems.
Proper response to the subscriber-originated signals normally requires the exercise of headend control over the various remotely located subscriber terminals coupled to the CATV system. The exercise of such control requires the downstream transmission of various command signals from the headend controller to the subscriber terminals.
The present invention provides a facility whereby such headend control of subscriber terminals may be conventiently realized within the environment of a prior art CATV system employing VBI data transmission techniques. In particular, a headend command signal is transmitted during a single VBI in the form of a multibit code, each bit of which is represented by the transmission or non-transmission of a subscriber address code during a respective predetermined time interval of the VBI. At the subscriber's terminal, the received address codes are processed to reconstruct the multibit code which is subsequently decoded for use in controlling various aspects of the terminal.